This invention relates to a fuel cell system and more particularly to a system having a plurality of cells which consume an H2-rich gas to produce power.
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cells gaseous reactants over the surfaces of the respective anode and cathode catalysts. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack and are commonly arranged in series. Each cell within the stack comprises the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells, hydrogen (H2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2). The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within a chemical fuel processor or reformer. The fuel processor contains one or more reactors wherein the fuel reacts with steam and sometimes air, to yield a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide. In reality, carbon monoxide and water are also produced. In a gasoline reformation process, steam, air and gasoline are reacted in a fuel processor which contains two sections. One is primarily a partial oxidation reactor (POX) and the other is primarily a steam reformer (SR). The fuel processor produces hydrogen, carbon dioxide, carbon monoxide and water. Downstream reactors may include a water/gas shift (WGS) and preferential oxidizer (PROX) reactors. In the PROX, carbon dioxide (CO2) is produced from carbon monoxide (CO) using oxygen from air as an oxidant. Here, control of air feed is important to selectively oxidize CO to CO2.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in co-pending U.S. patent application Ser. Nos. 08/975,422 and 08/980,087, filed in November, 1997, and U.S. Ser. No. 09/187,125, filed in November, 1998, and each assigned to General Motors Corporation, assignee of the present invention; and in International Application Publication Number WO 98/08771, published Mar. 5, 1998. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, and assigned to General Motors Corporation.
Efficient operation of a fuel cell system depends on the ability to effectively balance the three main components of a fuel cell system, namely, the combustor, the fuel cell stack and the fuel processor. While this hardware arrangement forms an efficient fuel cell system, it creates a control balance problem since the three major components have circular dependencies. Specifically, the combustor receives excess fuel from the stack. The stack receives fuel from the fuel processor and the fuel processor receives heat from the combustor. Control problems may arise if all three components cannot maintain a perfect balance. For example, if the combustor sends too little heat to the fuel processor, the fuel processor cannot fully reform water and methanol such that the fuel cell stack receives harmful xe2x80x9cbreakthroughxe2x80x9d methanol. Conversely, if the combustor sends too much heat to the fuel processor, the fuel processor will overheat and damage the fuel processor catalyst.
Other system constraints add to the control problem. The fuel cell stack requires a specific amount of reformate from the fuel processor. The amount depends on the vehicle load and its operating anode lambda. Typically, the anode lambda is about 1.2 meaning that the stack receives 1.2 times the hydrogen needed for the amperage requested from the vehicle. The stack consumes the hydrogen to generate electricity, with any excess hydrogen sent to the combustor.
In a typical implementation, the fuel processor converts methanol and water into reformate consisting of hydrogen, carbon dioxide and water. The reformate feeds into the anode side of the fuel cell stack. Thus, the fuel processor must deliver the exact amount of reformate to the stack. Supplying more reformate than necessary hurts efficiency. Supplying less reformate than necessary does not meet the needs of the vehicle.
The combustor uses excess stack hydrogen as fuel and combusts it with air to generate heat for the fuel processor. The fuel processor needs the heat to convert methanol and water into reformate. However, the combustor must consume all of the excess hydrogen received from the fuel cell stack. Otherwise, flammable hydrogen passes outside the fuel cell system during normal operation.
Operating conditions that do not allow a perfect balance require an adjustment. Thus, it would be desirable to provide an apparatus and method for efficiently heating the fuel processor of a fuel cell system which allows adjustment in the operation of the interconnected combustor, fuel cell stack and fuel processor of the fuel cell system. It would also be desirable to provide an apparatus and method which provides these functions while at the same time meeting the vehicle load needs during dynamic fuel cell system operation and preventing the expulsion of flammable hydrogen outside of the fuel cell system.
The present invention is a unique control apparatus and method for efficiently controlling the amount of heat generated by a fuel processor which uniquely controls a combustor fuel injector and an excess heat dump valve such that only one of the fuel injector or the heat dump valve is on at any given time.
In one aspect of the invention, the unique method includes the steps:
determining a temperature difference between the actual operating temperature of the fuel processor and the desired operating temperature of the fuel processor at a given fuel cell stack load;
if the actual temperature of the fuel processor is less than the desired temperature of the fuel processor, predicting the amount of heat required by the fuel processor based on the determined temperature difference;
generating a command to the fuel injector to supply fuel to the combustor to generate the required amount of heat only if the heat dump valve is closed;
if the actual temperature of the fuel processor is greater than the desired temperature of the fuel processor, determining the amount of heat that must be diverted from the combustor prior to input to the fuel processor to lower the actual temperature of the fuel processor to the desired temperature;
determining the position of the heat dump valve orifice to divert the determined amount of heat; and
generating a command to the heat dump valve orifice to divert the desired amount of heat only if the fuel injector is closed.
In another aspect of the invention, the method further comprises the steps of determining if the fuel injector orifice is open and, if so, generating a signal; and determining if the heat dump valve orifice is open and, if so, generating a signal.
Preferably, the method includes the step of providing the heat dump valve as linear actuator with a variable diameter outlet or orifice.
The method also maintains the heat dump valve orifice open until the actual temperature of the fuel processor equals or is less than the desired operating temperature of the fuel processor.
The method also includes the step of maintaining the fuel injector on until the actual temperature of the fuel processor is greater than or equal to the desired operating temperature of the fuel processor.
Finally, the method of generating the required amount of heat comprises the steps of:
generating a required heat command; and
converting the required heat command to the amount of fuel injected to the fuel injector based on air flow to the combustor.
In another aspect of the invention, an apparatus is provided for controlling the amount of heat generated by a fuel processor in a fuel cell system, the apparatus includes means for predicting the heat required by the fuel processor for a given load on the fuel cell stack, and means for determining a temperature difference between the actual operating temperature of the fuel processor and the desired operating temperature of the fuel processor at a given load. Means are provided for obtaining a desired heat value based on the predicted heat and the temperature difference.
The apparatus also includes means for converting the desired heat value to a desired fuel injector flow signal. Means are provided for determining a heat dump valve flow position for a desired amount of heat diversion. Finally, the apparatus includes means, responsive to the state of the heat dump valve and the fuel injector, for preventing the heat dump valve from opening if the fuel injector is opened and for preventing the fuel injector from opening if the heat dump valve is opened.
The control apparatus and method of the present invention uniquely maintains the operating conditions and parameters of a combustor, fuel cell stack and fuel processor in a fuel cell system in a balanced state under all load conditions by efficiently controlling the heating of the fuel processor. This balanced state is achieved by preventing an excess heat dump valve and a combustor fuel injector from being activated at the same time. In this manner, heat trim to increase the heat supplied to the fuel processor or a heat dump to divert a portion of the heat generated by the combustor away from the fuel processor ensures that the various elements of the fuel cell system remain in a balanced state.